Coalescing filters are used for a variety of applications. In general, gas coalescers serve to remove aerosol contaminants, both liquid and solid, from gaseous streams, for example, in purifying compressed gases such as air, helium, hydrogen, nitrogen, carbon dioxide, and natural gas, and in filtering inert gases used in recovering oil. They also may be used to collect liquid aerosol contaminants, such as in the filtering of vacuum pump exhausts where they serve to both prevent contamination of the environment and reclaim expensive vacuum pump oil. Similarly, they may be used in filtering chemical mists from low pressure chemical process streams to prevent pollution by, and to reclaim, liquid chemical aerosols.
Typically, coalescing filters are relied upon to remove the most difficult to separate aerosols. For example, oil lubricated compressors are widely used for compressing gases. Such compressors produce aerosols as a result of mechanical shearing and a combination of oil vaporization and subsequent downstream condensation. The aerosols formed generally comprise particles ranging in size from about 0.01 to about 50 micrometers.
Water aerosols are formed when the intake gas to a compressor contains sufficient water vapor that the resulting compressed and cooled gas exceeds 100 percent relative humidity. This commonly occurs, e.g., when the intake gas is atmospheric air or when the gas to be compressed is process gas that has come in contact with water.
Highly hydrophobic filters, also referred to as "barrier filters", are sometimes used to remove water-based aerosols of relatively large particle size from gas streams. These filters work by preventing water from passing through the filter medium by trapping the water-based aerosols on the upstream surface of the medium. The pores of such barrier filters must be smaller than the aerosol particles being removed. Accordingly, they are not efficient for removal of small aerosol particles since the pressure drop would be prohibitive.
Larger aerosol particles (larger than about 0.6 micrometers) tend to impinge and coalesce on surfaces throughout piping systems because their momentum often is too great to follow the flow path. These larger particles may be removed and, for economic reasons, generally are removed by other separating means, e.g., after-coolers and centrifugal separators or demisters. Coalescing filter elements, however, typically must be relied upon to remove aerosol particles ranging in size from about 0.1 to about 0.6 micrometers. Such aerosols are considered to be the most difficult to separate because they display marginal impactive removal and do not have sufficient diffusional characteristics to divert from system flow to allow interaction with separating devices. Smaller aerosol particles, e.g., less than about 0.1 micrometer, typically can be removed with somewhat coarser filters because they rapidly diffuse to surrounding surfaces.
Whether or not other separating means are used, coalescing filters conventionally are designed with well-known principles in mind. For a coalescing filter having a given voids volume, and for a gaseous stream having a given flow rate and aerosol loading, the filtering efficiency generally increases as the pore size decreases and/or the thickness of the filter medium increases. Decreasing pore size and/or increasing filter medium thickness, however, increases the pressure drop across the medium and, thereby, the energy required to maintain a given flow rate. With a given volume or space constraint, e.g., a filter cartridge of specified size, the use of a thicker filter medium will generally result in limiting available filter surface area, an increase in overall flow velocity, and correspondingly higher flow resistance. Increased flow velocity through the filter also decreases separation efficiency for the difficult-to-remove-size aerosol particles.
In the past, coalescing filter media have been designed to trade-off and optimize these competing factors. Many filter media so designed offer good performance under dry conditions. Eventually, however, the medium accumulates liquid as a result of collecting liquid aerosol particles. The accumulated liquid tends to block the smaller pores and thereby reduce the filtering efficiency of the medium. Blocked pores also increase the pressure drop across the medium which, in turn, increases energy requirements.
Conventional coalescing filters not only exhibit reduced efficiency and increased pressure drops when wet, they also tend to produce secondary aerosols. These can be formed by two mechanisms. As the smaller pores are blocked, the velocity of gas through the larger unblocked pores increases. The increased velocity increases the likelihood of shearing liquids from the surface of the filter medium and forming secondary aerosols downstream of the filter. Additionally, as the coalesced liquid flows down the filter, it can form a film over the pores. Gas passing through the filter tends to expel the liquid by forming bubbles which burst, forming secondary aerosols downstream of the filter. In short, the overall performance of the filter medium suffers when wetted.
At least some solid particulate matter is generally present in gaseous streams being treated for removal of liquid aerosols. Such dirt may be associated with the gas to be filtered or can arise as a result of wear and corrosion of the system apparatus. This solid particulate matter will also block pores in the filter medium and contribute to increased pressure drop.
For the most efficient separation of entrained aerosols from a gas stream then, a high performance coalescing filter should have the following characteristics:
(1) a high separation efficiency for aerosols having a particle size of from about 0.1 to about 0.6 micrometers over a wide range of influent concentrations, (as noted, these aerosols are considered to be the most difficult to separate);
(2) a low pressure drop (flow resistance) under wet conditions since flow resistance should be minimized in order to reduce energy losses;
(3) the ability to continue effective operation when handling and collecting liquid aerosols, while avoiding secondary aerosol formation downstream of the filter by virtue of shearing forces as the gas passes through the filter or by the "blowing of bubbles" as the pores are bridged by the coalesced fluid and subsequently blown out under the pressure of gas passing through the filter; and
(4) a high dirt-holding capacity to accommodate solids accumulation, while retaining low pressure drop, i.e., a coalescing filter must be able to handle effectively the dirt-loading which may result from the intake of particles resulting from corrosion and wear which may be released into the gas stream being filtered.
In order to evaluate the liquid aerosol separation efficiency and saturated pressure drop (.DELTA.P) of high efficiency coalescing filter media, Pall Corporation developed a test method described in detail in its November 1984 publication PEDD-FSR 101a entitled "Field Report 101, Practical In-Service Simulation Tests For The Rating Of High Efficiency Aerosol Coalescing Filter Performance".
The subject invention is directed to coalescing filters which maintain high efficiency and low pressure drop even when operating under wet conditions and which in large measure have the characteristics (1)-(4) set out above.